ACPI / debugger: Fix regression introduced by IS_ERR_VALUE() removal

The FIFO unlocking mechanism in acpi_dbg has been broken by the
following commit:

  Commit: 287980e4
  Subject: remove lots of IS_ERR_VALUE abuses

It converted !IS_ERR_VALUE(ret) into !ret which was not entirely
correct. Fix the regression by taking ret > 0 into account too as
appropriate.

Fixes: 287980e4 (remove lots of IS_ERR_VALUE abuses)
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
[ rjw: Simplifications, changelog & subject massage ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpi_dbg.c

@@ -602,7 +602,7 @@ static int acpi_aml_read_user(char __user *buf, int len)
 	crc->tail = (crc->tail + n) & (ACPI_AML_BUF_SIZE - 1);
 	ret = n;
 out:
-	acpi_aml_unlock_fifo(ACPI_AML_OUT_USER, !ret);
+	acpi_aml_unlock_fifo(ACPI_AML_OUT_USER, ret >= 0);
 	return ret;
 }
 
@@ -672,7 +672,7 @@ static int acpi_aml_write_user(const char __user *buf, int len)
 	crc->head = (crc->head + n) & (ACPI_AML_BUF_SIZE - 1);
 	ret = n;
 out:
-	acpi_aml_unlock_fifo(ACPI_AML_IN_USER, !ret);
+	acpi_aml_unlock_fifo(ACPI_AML_IN_USER, ret >= 0);
 	return n;
 }